Process of web drying



y 1956 c. P. BRITTON ETAL 2,746,167

PRQCESS OF WEB DRYING 2 Sheets-Sheet 1 Filed NOV. 50, 1954 INVENTORS E R BRI TTON CARLYL JOHN T. EIKER 0 EARL 11 E 3 ROBERT M. LESTER, JR

ATTORNEY y 1956 c. P. BRITTON ETAL 2,746,167

PROCESS OF WEB DRYING Filed NOV. 30, 1954 2 Sheets-Sheet 2 fig. 2

TRANSVERSE MOISTURE DISTRIBUTION PROFILE AT CERTAIN POINTS IN DRYER at entrance to dryer Edge| Edge M at tension release section I I I l at tension increase section WEIGHT OF WATER IN FILM WEIGHT OF DRY CELLULOSE IN FILM E5 I i I at exitof dryer WIDTH OF FILM INVENTORS CARLYLE P. BRITTON JOHN T EIKER 3RD EARL T. ELLIS ROBERT M. LESTER, JR.

BY %WQ/ e4.

ATTORNEY United States Patent Oflice 2,746,167 Patented May 22, 1956 PROCESS OF WEB DRYING Carlyle P. Britton, Richmond, Va., John T. Eiker 3d, Passaic, N. 3., and Earl T. Ellis, Petersburg, and Robert M. Lester, .lr., Richmond, Va., assignors to E. 5. du Pont de Nemours & Company, Wilmington, Del., :1 corporation of Delaware Application November 30, 1954, Serial No. 471,984

4 Claims. (Cl. 34-41) This invention relates to the manufacture of continuous, non-fibrous cellulosic sheet material which is subject to shrinkage during drying. More particularly, it relates to the drying of regenerated cellulose film so as to obtain a fiat sheet.

In the continuous manufacture of regenerated cellulose film, it is essential that it be dried in such a manner as to give a dry film which is substantially fiat. The problem of obtaining a flat sheet is a very difiicult one since regenerated cellulose film shrinks in all three dimensions when it is being dried, thus setting up nonuniform tension conditions within the film.

Flatness of a film is a characteristic of continuous films which is an important determining factor in their running qualities on converter and processing equipment. An absolutely fiat sheet, having no droop, is the goal of all continuous film manufacturers.

Flatness is determined by stretching a film longitudinally across two carefully aligned, horizontal, parallel supporting rolls or bars 15 feet apart. The film ends are rigidly held and tension is applied longitudinally to the film over the unsupported span of 15 feet. The contour of a transverse section of the film at approximately the center of the unsupported span is observed and major departures from a straight, horizontal line are measured in inches. Experience has shown that a lack of flatness and, particularly, the presence of slack longitudinal lane between two supporting lanes, called center droop, is extremely detrimental to satisfactory operation in bag-making machines, wrapping machinery, printing presses, etc. Drooping of the film edges, depending upon severity, is also undesirable.

Much prior work has been done with the objective of producing an essentially flat film web, no lanes of which differ in length by an amount which will cause droop. However, none of the processes heretofore developed have given entirely satisfactory web flatness characteristics, and have thus never been adopted in commercial operation.

Additional characteristics of regenerated cellulose which the prior art has sought to improve without notable success are those of permanent shrinkage, and deformation. Regenerated cellulose film as usually made in commerce is characterized by internal strains which, over a period of time, give rise to an irreversible shrinkage or loss in dimensions during use. Thus, when a box of candy, for example, is wrapped in a film containing the above described internal strains, it will be found that over a period of time, for instance, one week to several months or more, the wrapper will gradually contract and tighten up around the box, thus impairing the otherwise neat appearance of the package and frequently breaking or shattering the wrap itself, and destroying its entire effectiveness. This irreversible contraction manifested as above is greatly accelerated if at some time during the life of the sheet it is exposed to high humidity conditions, the sheet absorbs moisture, seeming to effect a much more rapid release of the strains so that if the sheet next becomes exposed to lower humidities, it will thereupon contract to smaller dimensions than it initially possessed. This irreversible shrinkage is termed permanent shrinkage, and is to be distinguished from deformation, which is the reversible change in dimensions which a sheet of regenerated cellulose undergoes with changes in its internal moisture content. Deformation manifests itself only upon changes in moisture content of the sheet while the process of relieving internal strains, which results in permanent shrinkage, takes effect more quickly on increasing the moisture content of the film, but also takes efiect at other or constant moisture contents. The following test illustrates the two phenomena: A sample of regenerated cellulose film is brought to equilibrium with an atmosphere of 35% relative humidity (R. H.) and 24 C. and measured. It is then brought to equilibrium with an atmosphere of %-100% R. H. and 35 C., with an atmosphere of 0%-5% R. H. and 35 C., and then back to 35 R. H. and 24 C. The difference in the length of the sample between the high and the low humidities measures the deformation and the difference between the initial and final lengths at 35 R. H. measures the permanent shrinkage.

An object of this invention, therefore, is to provide a process for the production of substantially flat, continuous, non-fibrous, cellulosic sheet material. A further object is to provide a process for continuously producing substantially fiat regenerated cellulose film having improved permanent shrinkage and deformation characteristics. A still further object is to provide an improved process for drying continuous gel regenerated cellulose film to produce a substantially fiat film having improved permanent shrinkage and deformation characteristics. The foregoing and other objects will more clearly appear hereinafter.

These objects are realized by the present invention which, briefly stated, comprises passing a continuous web of non-fibrous, gel (water-saturated) cellulosic film, e. g., gel regenerated cellulose film, through a drying zone wherein the web is subjected to drying conditions maintained substantially uniform across the width of the web, whereby the web transverse moisture distribution profile throughout the drying process is substantially symmetrical, maintaining the web under machine direction (longitudinal) tension just suflicient to prevent wrinkling or distortion of the web until the moisture content of the edge lanes of the web has been reduced to approximately 1.5 to 2.5 times the weight of the dry cellulose, and the moisture content of the center section of the web is greater than the moisture content of the edge lanes thereof, thereafter reducing the machine direction tension on the web and maintaining the web under reduced tension until the moisture content of the edge lanes has been reduced to approximately 0.1 to 0.5 times the weight of the dry cellulose, and the moisture content of the center section has been simultaneously reduced to approximately 0.3-1.5 times the weight of the cellulose, and thereafter increasing the machine direction tension on the web, by an amount sufiicient to elongate the more moist and still plastic shorter center section of the web to approximately the same length as the drier edge lanes of-the web and completing drying thereof.

By the expression tension just sufiicient to prevent wrinkling or distortion of the web, used herein, is meant, a tension on the web, in the machine or longitudinal direction of the web, which is effective to prevent wrinkling or distortion of the web, but which is not sulficient to prevent substantial shrinkage of the web as it dries.

By the expression "web transverse moisture distribution profile, used herein, is meant a plot of the moisture content across the width of the web (transverse axis of the web) measured by analyzing the moisture content of successive bands or lanes across the web, each band being three inches wide.

By edge lanes is meant the areas at each edge of the web extending lengthwise of the web and each bounded on one side by the edge of the web and on the other side by a line from'3 to 6 inches in from the edge of web and parallel thereto. The center section constitutes the remaining area of the web between the edge lanes.

The'process of the present invntion is generally applicable to the drying of continuous webs of any Wet-cast cellulosic film. However, because of the commercial importance of regenerated cellulose film derived from viscose, such film constitutes the preferred material for treatment in accordance with the principles of the present invention; and the invention will be described hereinafter with specific reference to the drying of gel regenerated cellulosic film.

The commercial scale method of continuously manufacturing regenerated cellulose film from an aqueous solution of sodium cellulose xanthate (viscose) is disclosed in the United States Patents Numbers 1,548,864 and 1,601,289 to Brandenberger. In this process, viscose is forced through an elongated orifice in the form of a sheet into a coagulating bath to form a coherent web, which freshly coagulated film is then promptly regenerated, washed, desulfured, bleached, softened and dried. The dried film is wound into rolls (sometimes called mill rolls) for convenience in handling, storing, and processing. Up through the softening stage, the web of regenerated cellulose remains in a gel (completely saturated with aqueous solution) condition. Since gel regenerated cellulose film contains about 3.0 to 3.5 times the weight of the dry cellulose as water, a substantial amount of moisture must be removed in the drying operation. in losing this moisture, the film shrinks in length, width and thickness (thus diminishing the area of the film obtained) and becomes puckered and wrinkled. To prevent excessive loss of area, the formation of surface irregularities (puckers, wrinkles and the like), and impairment of transparency, it is customary to dry the web by passing it about a series of heated rolls, such as are described in United States Patent No. 2,000,179 (Herndon), United States Patent No. 2,141,277 (Chylinski), etc., which are operated at speeds which maintain in the film both longitudinal and transverse tensions sufficient to lessen or prevent the aforementioned decrease in area and surface deformation. The present invention is concerned with the drying stage of the above described process of manufacture and will now be specifically dis-. closed with reference to the accompanying drawings wherein:

Figure 1 is a fragmentary diagrammatic elevational view of drying apparatus; and

Figure 2 is a graph showing the web transverse moisture distribution profile at indicated points in the drying apparatus.

Referring to Figure 1, wet gel regenerated cellulose film web F, after leaving the softener tank, passes between squeeze rolls or scraping devices 1 and 2 which remove excess softener bath, and enters the first section of the dryer at approximately the same linear speed as in the softener tank and contains approximately 3.0-3.5 times the weight of the dry cellulose as water. In the first stage of the dryer, the film web passes over a series of conventional dryer rolls 3 operated at a speed which will maintain the web under just enough longitudinal or machine direction tension to prevent wrinkling or distortion of the web. The dryer rolls throughout the dryer are uniformly heated and heated air is circulated across the web so as to maintain the web at substantially uniform drying temperature whereby to insure a symmetrical transverse moisture distribution profile (see Figure 2). When the moisture content of the edge lanes of the film web has been reduced to approximately 1.5-2.5 times the weight of the dry cellulose, with the web transverse moisture distribution profile being substantially symmetrical and convex (edge lanes drier than the center section), the web is passed over a second series of conventional dryer rolls 4 (comprising the second stage of the dryer) driven at a peripheral speed of 0.5% to 2% lower than that of the preceding rolls to effect a decrease in, or a relaxing of the tension on the web which has increased as a result of shrinkage forces generated by the drying film which is still traveling at the same linear speed. In the second stage of the dryer, the moisture content of the edge lanes is further reduced to 0.l-0.5 times the weight of the dry cellulose and the moisture content of the center section of the Web is reduced to approximately 0.3l.5 times the weight of the dry cellulose (the web transverse moisture distribution profile being symmetrical and convex) after which the web is passed through the third stage of the dryer comprising a series of conventional dryer rolls 5 drawn at a peripheral speed of from 0.5% to 3.0% greater than that of the rolls of the secstage of the dryer. This increase in peripheral speed in creases web tension and causes the more moist and still plastic shorter lanes to elongate to approximately the same length as the already-dried edge (or other longer) lanes. The drying is then completed with all film lanes at approximately the same length, and the film leaves the dryer and is collected as a mill roll 6 with a moisture content of approximately 0.06 times the weight of the dry cellulose. As stated hereinabove, at the critical stages where the peripheral speed is decreased and then later increased, the web transverse moisture distribution profile must be approximately symmetrical and the edge lanes must always be drier than the center. This is achieved by maintaining the drying temperatures substantially uniform across the faces of the web.

The optimum decrease and increase in peripheral speeds differ somewhat for different thicknesses of film, and for films with different degrees of plasticization. For this reason it is desirable, but not essential, to be able to vary the difference between peripheral speeds before and after the points of critical web moisture content. Excellent sheet flatness has been obtained with variable and with fixed speed differences, and with the following methods of applying the difference:

1. At one point between two successive dryer rolls.

2. incrementally, with the speed differential distributed over from two or more lower dryer rolls.

3. Between the entrance and exit of a section of freely rotating rolls, driven by the web, the film moisture contents at entrance to this section being in the specified ranges.

Mill rolls of regenerated cellulose film dried under the conditions of this invention may be slit directly into rolls of narrower width as specified by the purchasers of the film, or the rolls may be coated with various compositions before being slit. As shown in the following tables, the slit rolls have excellent flatness properties, as well as improved deformation and permanent shrinkage characteristics.

Sheet flatness data of split rolls from mill rolls made according to the prior method of drying wherein all dryer rolls operate at approximately the same peripheral speed (identified in the table as the Old method) and'frorn TABLE I Comparative slit roll sheet flatness data It is readily seen that this invention gives slit rolls with a much higher percentage of acceptance than the old method, especially with film 0.0014" thick. Slit rolls having edge droop of more than A" at 1.4 lbs. of tension per inch of film width are considered unsatisfactory for use on such automatic machinery as printing presses, bagmaking equipment, etc.

Examples showing improvement in deformation and in permanent shrinkage are given in Table II.

TABLE II Deformation and permanent shrinkage RTiensionk e ease Control Tension Increase Transverse Direction:

Percent Deformation 5. 4 3.9 Percent Permanent Shrinkage 2. 9 3.0 Machine Direction:

Percent Deformation 3. 4 2. 1 Percent Permanent Shrinkage 3. 4 2. 8

We claim:

1. The process which comprises passing a continuous web of non-fibrous gel cellulosic sheet material, comprising edge lanes and a center section, through a drying zone; subjecting the web in said zone to drying temperatures maintained substantially uniform across the width of the web whereby the web transverse moisture distribution profile is maintained substantially symmetrical; maintaining the web under longitudinal tension just suflicient to prevent wrinkling or distortion of the web until the moisture content of the edge lanes of the web has been reduced to approximately 1.5 to 2.5 times the weight of the dry cellulosic material, said center section being more moist than said edge lanes; thereafter decreasing the longitudinal tension of said Web and maintaining said web under decreased tension until the moisture content of the edge lanes of the web has been reduced to approximately 0.1 to 0.5 times the weight of the dry cellulosic material and the moisture content of the center section of the web has been simultaneously reduced to approximately 0.3 to 1.5 times the weight of the dry cellulosic material, said center section being more moist than said edge lanes; and thereafter increasing the longitudinal tension on the web to an amount suificient to elongate the center section to approximately the same length as the edge lanes; and completing drying of the Web.

2. The process of claim 1 wherein the cellulosic sheet material is regenerated cellulose film.

3. The process which comprises passing a continuous web of gel regenerated cellulose film, comprising edge lanes and a center section, over a first series of dryer rolls driven at a speed effective to maintain the web under a longitudinal tension just sufiicient to prevent wrinkling or distortion of the web, said web being subjected to drying temperatures maintained substantially uniform across the Width of the web; drying said film on said first series of rolls until the moisture content of the edge lanes of said web has been reduced to approximately 1.5 to 2.5 times the weight of the dry cellulose, said center section being more moist than said edge lanes; thereafter passing said web over a second series of dryer rolls driven at a lower peripheral speed than that of said first series of dryer rolls whereby to reduce the longitudinal tension on said web; further drying said web on said second series of dryer rolls until the moisture content of the edge lanes of said web has been reduced to approximately 0.1 to 0.5 times the weight of the dry cellulose and the moisture content of the center section of said web has been simultaneously reduced to approximately 0.3 to 1.5 times the weight of the dry cellulose, the center section being more moist than the edge lanes; and thereafter passing said Web over a third series of dryer rolls driven at a speed eifective to increase the longitudinal tension on the web to an amount sulficient to elongate the center section of said web to substantially the same length as the edge lanes; and completing drying of said webs in said third series of dryer rolls.

4. The process of claim 3 wherein said second series of rolls is driven at a peripheral speed between about 0.5% to about 2% below the peripheral speed of said first series of rolls, and said third series of rolls is driven at a peripheral speed between about 0.4% and about 3.0% above the peripheral speed of said second series of rolls.

References Cited in the file of this patent UNITED STATES PATENTS 1,975,708 Bleibler Oct. 3, 1934 2,115,132 Alles et a1 Apr. 26, 1938 2,308,161 Eckstein et al Jan. 12, 1943 

1. THE PROCESS WHICH COMPRISES PASSING A CONTINUOUS WEB OF NON-FIBROUS GEL CELLULOSIC SHEET MATERIAL, COMPRISING EDGE LANES AND A CENTER SECTION, THROUGH A DRYING ZONE; SUBJECTING THE WEB IN SAID ZONE TO DRYING TEMPERATURES MAINTAINED SUBSTANTIALLY UNIFORM ACROSS THE WIDTH OF THE WEB WHEREBY THE WEB TRANSVERSE MOISTURE DISTRIBUTION PROFILE IS MAINTAINED SUBSTANTIALLY SYMMETRICAL; MAINTAINING THE WEB UNDER LONGITUDINAL TENSION JUST SUFFICIENT TO PREVENT WRINKLING OR DISTORTION OF THE WEB UNTIL THE MOISTURE CONTENT OF THE EDGE LANES OF THE WEB HAS BEEN REDUCED TO APPROXIMATELY 1.5 TO 2.5 TIMES THE WEIGHT OF THE DRY CELLULOSIC MATERIAL, SAID CENTER SECTION BEING MORE MOIST THAN SAID EDGE LANES; THEREAFTER DECREASING THE 